Solvents

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Most industrial processes use solvents for cleaning manufactured articles. These include a number of ozone-depleting ones. The latter are principally:

carbon tetrachloride
CFC-113
1,1,1-trichloroethane
HCFC-141b

along with a number of smaller contenders. These are often sold under trade names.

Carbon tetrachloride (CTC)

Because its use as a solvent was (and still is) very seriously underestimated by the Parties to the Montreal Protocol, it was initially not considered a very serious problem. In fact, this solvent is a very bad ozone-depleter, with an ODP of 1.1, and we do not know, even today, how much is being used for this purpose. The problem is that the substance is also legally used as feedstock for making a wide range of other chemicals; as it is theoretically totally transformed to non-ozone-depleting chemicals in this process, feedstock is not regulated under the Montreal Protocol. It is also used as a process agent. It is too easy to divert some from one use to another.

What is known is that there has been practically no solvent use of CTC since about 1960 in developed countries, because of its toxicity and possible but unproved carcinogenicity in humans. Because of this, use in developing countries was initially thought to be minimal, but this proved to be totally wrong. I have seen CTC being used in more than one country under the most horrific conditions, literally endangering the lives of operators and even third parties by infiltration into water supplies, as recently as 2003.

Why has this been allowed to happen? Simply because of ignorance and because, of all the non-flammable solvents, it is the cheapest and amongst the most effective for general degreasing.

It is easy to phase out nearly all solvent uses of CTC. In most cases, simple substitution by the less toxic trichloroethylene (TCE), which does not deplete the ozone layer, is an easy and economical first step. However, TCE has been wrongly accused of carcinogenicity by some ill-informed authorities and, for this reason, some people are wary of using it. By maintaining an OEL of 50 ppm, there is no known risk of health effects with TCE and certainly no risk of cancer. That having been said, in a second phase, it may be desirable to switch to chemicals or processes with a higher OEL because, although TCE is much less toxic than CTC, it is still toxic and exposure control is necessary.

CFC-113

CFC-113 is a useful solvent that was much used for defluxing in the electronics industry and precision cleaning. It has an ODP of 0.8 and, because of this high value and its popularity, was responsible for about 12% of all ozone depletion and 60% within the solvents sector, prior to 1987. No direct single substitute has been identified, but a wide choice of other solvents and processes means that it can be easily phased out in nearly all applications. It is this wide choice that has embarrassed some users who have been unable to select which one for their specific application, especially in developed countries, where it is still widely used.

Because of its low toxicity, it has always been popular with Small and Medium Enterprises (SMEs), able to use it in cheap machines which cause high emissions. This is probably the biggest obstacle today in phasing the substance out in developing countries, because there are literally many thousands of SMEs which are almost impossible to identify through long and convoluted sales chains. I estimate that well over 50% of the remaining solvents sector ozone-depleting substance emissions come from this source and this possibly represents as much as 25 or 30% of emissions from all sectors combined. Unfortunately, there is still much work to be done here.

1,1,1-Trichloroethane (TCA)

TCA, also popularly but incorrectly known as methyl chloroform, was the most popular general degreasing solvent prior to 1987, with a few minor additional applications. It has an ODP of 0.1 which is low, but the vast volumes used accounted for over 30% of all ozone depletion in the solvents sector. It has almost identical characteristics to trichloroethylene (TCE) but, unfortunately, the latter is much more toxic than TCA which has somewhat limited its usefulness in direct substitution. Like CFC-113, there is a wide choice of substitutes available, with the same disadvantage of selection difficulty.

Because it has a fairly short Atmospheric Residency Time (popularly, but incorrectly, termed as half-life) the effective phase-out in developed countries has meant that the atmospheric concentration has already diminished, which is not the case with CFC-113.

HCFC-141b

This solvent was introduced onto the market in the late 1980s. It has characteristics very similar to TCA, including its ODP (0.11). As such, there is no advantage in using it as a substitute for TCA, and its use has been discouraged for this purpose. However, it does present small advantages as a substitute for CTC or CFC-113. Its use is already banned in many countries, such as the USA and the EU.

Other OD solvents

HCFC-225 is an expensive product with characteristics similar to CFC-113. Its ODP is officially classed as 0.02 to 0.07 and it therefore has a distinct advantage over CFC-113. However, it is more toxic than CFC-113 and reasonable care to avoid operator exposure is required. It is a blend of isomers.

HCFC-123 has also been proposed but its toxicity is a disadvantage, ruling it out for most applications, despite its very low ODP (0.02).

Bromochloromethane is a toxic and ozone-depleting substance with an ODP of 0.12. Its use is banned in all countries under the Montreal Protocol.

1-Bromopropane, n-propyl bromide or nPB is a special case, meriting a page to itself.

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